1. Field of the Invention
This invention relates to an alignment mark useful in the formation of pattern using typically a lithography technique in the process of manufacturing a semiconductor device, in particular to an alignment mark to be used in the formation or processing of a resist pattern and to the manufacturing method of the alignment mark.
Further, this invention relates to an exposing method using the above mentioned alignment mark and to a semiconductor device to be manufactured using the exposing method.
2. Description of the Related Art
In the manufacture of semiconductor device, miniaturization of unit structure (cell) constituting the device contributes to the saving of manufacturing cost of the device of high performance and large scale. In an effort for realizing the miniaturization of cell, the parties concerned to the manufacture of semiconductor device are making an extensive effort on the technique and research of semiconductor device.
In order to realize the fine working of the device cell, it is imperative to improve working techniques such as an anisotropic etching to the vertical direction. Additionally, a further development of technique with respect to fine resist-patterning such as an optical lithography is also demanded.
Regarding the formation of resist pattering, the resolution can be improved by the following techniques. First, it is desired to select as an exposure light a ray of shorter wavelength such as g-line, i-line, extreme ultraviolet ray. It is also possible to improve the resolution by using an improved irradiation method, such as a converted irradiation. Now, owing to the development of a photomask which capable of controlling not only the distribution in intensity of transmitted light but also the phase of light, the resolution of pattern is becoming more improved.
Additionally, it is also required in order to achieve a fine patterning a mask pattern over a pattern preliminarily formed on the surface of underlying substrate with accuracy. Namely, a high accurate alignment is required. For the purpose of achieving this high accurate alignment, alignment systems of various type have been developed.
There are two typical alignment systems, i.e., a system wherein the alignment is individually performed to each exposure area which has been formed in advance by dividing a substrate into several chips; and a system wherein the alignment is performed all over the substrate at first, and exposure is performed on all exposure area. According to the former system, each exposure area formed by dividing a substrate into several chips is provided with alignment marks, so that the alignment between a mask and a chip is performed by detecting the location of the alignment marks immediately before irradiating an exposure light to each chip. Subsequently, an alignment light for transferring the mark pattern to the substrate is sequentially moved to the exposure area of each chip thereby performing the exposure.
On the other hand, according to the latter system, the alignment mark is put on two or more locations of the substrate. In an operation of aligning a mask with a substrate, a stage carrying substrate is moved to a mark-detecting position to sequentially detecting the alignment marks, and then on the basis of the extent of movement of the stage, the alignment between the mask and the substrate as a whole is performed. Subsequently, the exposure all over the exposure area of substrate is performed.
In detecting the location of alignment mark, an optical detecting system instead of a needle-contacting system is generally employed. Accordingly, in this respect, the above mentioned two alignment exposure systems belong to the same optical alignment system.
An optical alignment system generally employed is an image processing method, which can be performed as follows. Namely, in one method an alignment light is first irradiated onto the regions of alignment marks on a mask and substrate. Then, the intensity profile of reflected light or transmitted light of the alignment light irradiated to the mask and substrate is measured with an area sensor. In another method, a mask alignment mark and a substrate alignment mark are first scanned with focusing alignment light. Then, an intensity change of scattered light of the alignment light scanning the mask mark and the substrate alignment mark is measured with a detector. From this intensity profile thus obtained, a waveform corresponding to the mark per se or the step of the mark is detected, and then the location of the mark is measured on the basis of the waveform. After measuring the locations of mask mark and the substrate (wafer) mark, the magnitude of relative misregistration between these marks is determined on the basis of the measured locations.
Recently, a new alignment method called "heterodyne method" using diffraction light of alignment light has been put into practical use. According to this heterodyne method, a mask and a substrate, each having a diffraction grid pattern or checkerwise lattice pattern as an alignment mark, are employed, and an alignment light is irradiated to the mask and substrate to detect a diffraction light. Based on the phase of this detected diffraction light, the magnitude of relative misregistration between the mask and substrate is measured.
These optical alignment methods may be applied to a substrate with high reflectivity such as a substrate covered with a metallic film such as aluminum film, a substrate having a transparent material layer on a metallic film mentioned above, or a substrate having alignment marks of a transparent material formed on a metallic film mentioned above. There is a problem however that when these optical alignment methods is applied to a substrate with high reflectivity as mentioned above, the alignment accuracy becomes much lowered as compared with a substrate with low reflectivity such as a LOCOS substrate or a gate substrate.
Specifically, in the case of a substrate with high reflectivity having a metallic film on its top surface, the reflectance of convex portion or concave portion constituting an alignment is almost the same as that in the periphery thereof, and the reflection intensity is highly influenced by the surface roughness of metallic film. Accordingly, when an image processing treatment is performed on a high reflectance substrate, only a reflection intensity profile of large noise can be obtained, causing a waveform corresponding to the step of the mark to be buried within the noise. Therefore, it is quite difficult to detect the waveform corresponding to the step of the mark from the reflection intensity profile, thus resulting in the lowering of alignment accuracy.
On the other hand, in the case of performing the positional detection from the data on intensity or phase of diffracted light, the changes of intensity or phase due to differences in the height of step or in cross-sectional shape may be caused to increase, if the difference in reflectance between the convex portion and the concave portion is small, thus giving rise to the generation of random offset portion in the measured value in each alignment mark.
Followings are explanations on the problem of lowering of alignment accuracy in a high reflectance substrate. As one example of using heterodyne method for a high reflectance substrate, X-rays proximity lithography was employed. In the heterodyne alignment method using X-rays proximity lighography, there is another problem in addition to the problems mentioned above that a multiple reflection of alignment light to be generated between the substrate and mask tends to become more conspicuous when a high reflectance substrate is used, thus making it one of reasons for the degradation of accuracy.
FIG. 1 illustrates a schematic view of an alignment system. FIG. 2 shows an incident light projected onto the marks of the mask and substrate; and a diffracted light from these marks.
In the alignment using the system shown in FIG. 1, HeNe laser beams 3a and 3b are projected onto the mask alignment mark 1 at same incident angle in XZ plane. Additionally, HeNe laser beams 4a and 4b are projected onto the substrate mark 2 at the same incident angle as mentioned above in XZ plane. Thereafter, the phase difference between a detecting light 5 diffracted at plus first order direction in YZ plane at the mask alignment mark and the detecting light 6 diffracted at plus first order direction in YZ plane at the substrate mark 2 is measured, and then the registration between the mask and the substrate is adjusted. Namely, an accurate measurement of a phase difference between the light 5 detected from the mask mark and the light 6 detected from the alignment mark leads to the improvement of alignment accuracy.
However, according to this system, multiple reflection 14 and 14a will be caused to generate between the mask 8 and the substrate 9 as shown in FIG. 2, so that disturbance light 11 and 12 will be generated. The disturbance light 11 is mingled with the light 5 detected from the mask mark and the disturbance light 12 is mingled with the light 6 detected from the alignment mark. If such a disturbance light is mingled with the detected light, the signal accuracy of alignment will be deteriorated.
The multiple reflection 14a can be vanished by providing a shade film 10 to the mask, so the mingling of the disturbance light 11 with the detected light 5 is prevented.
The influence of this disturbance light on the detected light 6 can be confirmed by measuring the noise to be generated when only GAP (a space between the mask and the substrate) is caused to change. Changing only of GAP can be effected for example by moving a substrate stage in Z direction while preventing the mask and substrate stages from being moved in XY direction. FIG. 3 illustrates the generation of noise at a cycle of .lambda./2 (.lambda.: wavelength of HeNe laser) in an alignment signal which originally should be indicating a constant value. When this amplitude in intensity of the detected light is converted to misregistration of alignment, it corresponds to 0.1 .mu.m or more of misregistration. From this fact, it can be seen that if an disturbance light is mingled with the detected light, it is no more possible to accurately measure the position of the mark.
In an ordinary exposure method other than the X-ray proximity lighography, it can be generally said that a delicate non-uniformity in the cross-sectional shape of alignment mark also tends to become a cause for the deterioration in alignment accuracy. This problem will be discussed below.
In the case of a high reflectance substrate, the reflectance of convex portion of an alignment mark is almost the same as that of concave portion because of its low transmittance. Additionally, the reflectance of the side wall of the mark also is almost the same with those of the convex and concave portions. Therefore, the phase of diffracted light is much influenced by the change in cross-sectional shape of the mark. In the heterodyne method where the detection of relative misregistration between the mask and the substrate is performed according to the phase, any change in cross-sectional shape of mark may be a cause of deterioration in alignment accuracy.
As explained above, when an alignment method generally employed in the formation of pattern is applied to a high reflectance substrate having a metallic film such as aluminum film deposited thereon, only an alignment of very poor accuracy is obtainable as compared with a low reflectance substrate according to the technical level of today. Therefore, in the manufacture of a semiconductor device using a metallic film such as aluminum film for the formation of metallic wiring, it is required for the purpose of improving alignment accuracy to take countermeasures such as thickening the wiring; increasing the number of wiring; or increasing the manufacturing steps for forming the wiring.
The problems pertinent to a high reflectance substrate have been explained in the above description. However, it is also difficult to obtain an alignment signal of high signal to noise ratio (S/N) in the case of performing an alignment between an underlying substrate of very low reflectance and a mask. Therefore, it is also difficult to obtain an alignment of satisfactory accuracy in this case.
In the occasion of applying an image processing treatment using a reflected light to an underlying substrate of low reflectance, an alignment light is first irradiated onto the substrate thereby obtaining a reflection intensity profile. Then, the location of mark is detected from the minimum value of reflection intensity appearing near the vicinity of the step of the alignment mark, thereby performing the alignment of it with the mask. However, in the case of low reflectance substrate, the reflection intensity of the background is very low so that its minimum value is caused to disappear, thereby making it difficult to detect even the location of mark.
As explained above, when the image processing treatment is applied to a high reflectance underlying substrate, the S/N ratio of alignment is caused to be much deteriorated as compared with other kinds of underlying substrate. In particular, in the case of the manufacturing process of semiconductor device, the formation of a metallic wiring would be very difficult or complicated.
On the other hand, in the case of low reflectance underlying substrate, it is conceivable, for the purpose of improving alignment accuracy, to improve the optical alignment system or the resolution of a detecting device. However, nothing have been successful to achieve such an improvement up to date.